A thin-film transistor (TFT) is a particular kind of field-effect transistor. The most common applications of TFTs are displays, such as LCD TVs and monitors. Less common is the use of TFTs as sensors, with the one exception being the use of TFT sensor arrays (with a scintillation layer) for the purposes of digital radiography (see for example DPIX, Inc., Palo Alto, Calif.). Whether for use in a display or a sensor application, each of these applications employ an array of pixels in a two-dimensional (2D) grid where within each pixel, layers of thin films (metals, oxides and amorphous silicon, for example) are deposited onto a substrate that is typically glass of 1.0 mm or thinner thickness. TFTs are created at each pixel (typically two or three per pixel) to facilitate the addressing of individual pixels whether for driving or readout purposes.
Present commercial optical fingerprint scanners capture images of reflected light representing an image of a fingerprint onto complementary metal-oxide semiconductor (CMOS) or charge-coupled device (CCD) two dimensional sensors. Such CMOS and CCD sensors are similar and sometimes identical to those used in commercial digital cameras. Free-space optics such as refractive lenses that comprise a multi-element objective lens are used to image the reflected light off of the finger and onto the 2D sensors. Because of the presence of free-space optics, a relatively large distance must be between the finger and the sensor, resulting in a fingerprint scanner that is larger and bulkier than would be desired for certain applications (in particular mobile, hand-held applications). It would be desirable to avoid free-space optics so as to provide significantly more compact and potentially lighter weight scanners than are presently utilized.